Method for energy dither of a particle beam

Abstract

A method for applying an energy dither to a charged particle beam in order to vary the wavelength of the charged particle beam. Bunches of charged particle beams are accelerated by cavities that are operated at a harmonic of the bunch repetition rate. One or more secondary radiofrequency accelerator cavities are added near the wiggler after the primary beam transport to apply a fluctuation between individual bunches with a pseudo-random distribution. The secondary radiofrequency accelerator cavities provide fine variations of the beam energy about a nominal operating point. Operating a free electron laser (FEL) with a 1% change in the electron beam energy via the secondary cavity will result in a 2% wavelength variation of the FEL output.

What is claimed is: 
     
       1. In a free electron laser (FEL) system including an input particle beam, a method for varying the output wavelength of the FEL output beam by changing the energy of the FEL input particle beam, comprising:
 a. providing a primary accelerator cavity for supplying the input particle beam and a primary beam transport for transporting the input particle beam, wherein the input particle beam has energy with a nominal operating point and a primary accelerator bunch frequency; 
 b. providing a wiggler to periodically deflect the beam of particles inside the input particle beam; 
 c. inserting one or more secondary radiofrequency accelerator cavities between the primary beam transport and the wiggler; 
 d. setting the frequency of each secondary radiofrequency accelerator cavity to a harmonic or sub-harmonic of the primary accelerator bunch frequency; and 
 e. dithering the particle beam energy of the FEL input particle beam by operating the secondary radiofrequency accelerator cavities at the harmonic or sub-harmonic of the primary accelerator bunch frequency, said dithering varying the particle beam energy of the FEL input particle beam about the nominal operating point and varying the wavelength of the FEL output beam.